NOTES ON THE ANATOMY OF STERNASPIS. 237 



staut structure (figs. 23 a and b). They consist of an outer 

 transparent sphere filled with clear liquid, in which some 

 minute granules are occasionally seen floating. In the centre 

 of the sphere is situated a liighly refringent yellowish body, 

 which I shall call the concretion. This body is composed of 

 two halves, one of which is slightly larger and darker than the 

 other. In each cell spheres may be found of varying sizes, 

 from a maximum diameter of about 15 f^i to a mere speck. 

 But even when quite minute they always contain, as far as I 

 have been able to see, a central concretion of structure similar 

 to that described above. When teased out and brought into 

 some foreign medium the spheres always burst and disappear ; 

 no fixative, as far as I am aware, will preserve them. The 

 concretions, on the other hand, are much more resistent. 

 Treatment with divers reagents reveals the fact that the two 

 halves have diff'erent properties, and, moreover, that they are 

 cup-shaped, enclosing a third element, which may be called 

 the central granule. Distilled water, ammonia, alcohol, and 

 ether have no eff'ect on the concretion. Caustic potash (5 per 

 cent.) dissolves the small half, and apparently the central 

 granule, but not the large half, which resists even when 

 heated. Acetic acid, on the other hand, dissolves the large 

 half, and the central granule after prolonged action ; the small 

 half remains unaff'ected. Weak hydrochloric acid dissolves 

 the large half first, and then the smaller, whilst the central 

 granule remains. Strong mineral acids destroy the whole 

 concretion. Neither osmic acid nor iodine stains the concre- 

 tion to any marked extent. 



Fig. 22 shows the smaller half and central granule (the 

 larger half having been dissolved) of concretions in cells pre- 

 served with Hermann^s fluid, and stained with alum carmine, 

 a stain which the concretions readily take up. 



Vejdovsky (5) briefly mentions the refringent granules in 

 the cells of the nephridium of Sternaspis ; but Rietsch curiously 

 enough seems to have mistaken them for nuclei.^ 



> " La couche epitheliale interne se compose de cellules tres inegales (fig. 

 52) : les unes, volumineuses, presenteut do uombreux noyaux dent la plupart 



